1. Field of the Invention
The present invention relates to an antenna apparatus used to transmit and receive wireless signals and, particularly, relates to an antenna apparatus having a patch antenna configuration in which a radiation conductor and a ground conductor plate are arranged so as to face each other with an insulating material disposed therebetween.
More particularly, the present invention relates to an antenna apparatus in which radiation of unwanted electromagnetic waves resulting from surface waves generated on an antenna substrate is suppressed, and distortion of a radiation pattern is thereby reduced and, particularly, relates to an antenna apparatus in which AMC (Artificial Magnetic Conductor) elements having resonance characteristics are mounted in the area surrounding a patch antenna unit.
2. Description of the Related Art
In wireless communication using a radio-wave communication method, signals are propagated by using a radiation electric field generated when electrical current is made to flow through an antenna. There are various types of antennas. In particular, examples of an antenna meeting the demand for a low-profile antenna include an antenna apparatus configured in such a manner that a radiation conductor and a ground conductor plate are arranged so as to face each other with an insulating material disposed therebetween, that is, a microstrip patch antenna (hereinafter will be simply abbreviated as a “patch antenna”).
FIG. 6 shows an example of the configuration of a patch antenna. For the shape of the radiation conductor plate, a rectangular shape as shown in the figure or a circular shape is used. For an insulating body, a dielectric is used, and the thickness thereof is approximately 1/10 of the wavelength of the wireless frequency or smaller; therefore the insulating body has a low profile. In actual manufacture, since the patch antenna is often manufactured by performing etching processing on a dielectric substrate, both sides of which are copper-clad, manufacturing is easy, and integration with a circuit substrate is easy.
According to the microstrip patch antenna having such a configuration, radiation directivity when it is excited in the lowest order mode (a TM10-mode in the case of a rectangular shape) generally indicates a single direction of a z-axis direction, and a directional gain of approximately several dBi is obtained. Furthermore, a power-feed point is provided at a position slightly offset from the center of the radiation conductor. As the electrical current components in an offset direction (that is, in an x-axis direction in the figure) increase, a radiation electric field is generated, and a standing wave is excited. Then, by adjusting the offset length, it is possible to achieve matching at 50 ohms.
Furthermore, a planar antenna has been proposed (see, for example, Japanese Unexamined Patent Application Publication No. 11-103213) in which, for example, a patch antenna unit is arranged so as to face a ground conductor unit with a dielectric provided therebetween, the center conductor of a coaxial cable is inserted from the opening of the ground conductor plate in such a manner as to go through the dielectric in the thickness direction thereof, the center conductor is electrically connected at a point P of the patch antenna unit, and radio waves are transmitted or received with the point P functioning as a power-feed point. When a coaxial cable is to be connected to the patch antenna unit, the center conductor of the coaxial cable can be directly inserted into the dielectric, and can be connected to the power-feed point with soldering or the like. Therefore, it is possible to simplify the antenna configuration and also possible to decrease the manufacturing cost.
Furthermore, it is possible to adopt a configuration in which an opening is provided in the ground conductor plate, and power feeding is performed in an electromagnetically coupled manner through the opening from the back side of the ground conductor plate.
A planar antenna, such as a patch antenna, has problems that a surface wave (an electromagnetic wave propagated on the surface of a ground conductor plate) occurs on an antenna substrate, the surface wave is propagated to the end portion of the antenna substrate, and an unwanted electromagnetic wave (an unwanted electromagnetic wave resulting from a surface wave) is radiated from the end portion of the antenna substrate, causing a radiation pattern radiated from the antenna to be distorted. Another problem is that an unwanted electromagnetic wave resulting from a surface wave is radiated to a circuit substrate disposed in the surrounding area and another antenna substrate, whereby radio interference occurs, and malfunction of a semiconductor element occurs.
With regard to the above problems, a solving method of disposing a mechanism for suppressing the propagation of a surface wave on an antenna substrate has been known. As a mechanism for suppressing surface-wave propagation, there is a mechanism called a high impedance surface or artificial magnetic conductor (hereinafter will be simply abbreviated as an “AMC”). For example, by periodically arranging AMC elements having resonance characteristics on a ground conductor plate, it is possible to suppress the propagation of a surface wave.
FIG. 7 shows an example of the configuration (sectional view) of a planar antenna utilizing AMC elements (see, for example, U.S. Pat. No. 6,262,495, and Dan Sievenpiper, et al. “High-Impedance Electromagnetic Surfaces with a Forbidden Frequency Band” (IEEE Transactions on Microwave Theory And Techniques, Vol. 47, No. 11, pp. 2059-2074)). Individual AMC elements are of a thumbtack-type in which a plate-shaped conductor is supported by a post-shaped conductor. By arranging many thumbtack-type AMC elements in the area surrounding the patch antenna, propagation of a surface wave that reaches the end portion of the ground conductor and causes unwanted radiation (scattering in which an edge is a secondary-wave source point) is suppressed. By suppressing excessive unwanted radiation, the effect of increasing the gain in a desired direction (towards the front of the patch antenna, in the upward direction in the plane of FIG. 7) is expected.
Although it is difficult to see from FIG. 7 because FIG. 7 is a sectional view, thumbtack-type AMC elements in which a plate-shaped conductor is supported by means of a post-shaped conductor are periodically arranged in a two-dimensional manner in the area surrounding a patch antenna. Then, resonance is caused to occur by inductance components by the post-shaped conductor and capacitance components with the plate-shaped conductor. As a result, the propagation of the surface wave that occurs in the patch antenna disposed in the center to the peripheral edge is suppressed.
However, in practice, electromagnetic simulation performed by the inventors of the present invention revealed that a frequency exists at which, if thumbtack-type AMC elements having the above-described resonance characteristics are arranged in the area surrounding the radiation conductor plate, the gain is decreased. An AMC element is designed to suppress propagation of a surface wave that flows toward the end portion of the ground conductor. It is considered that a main reason for a decrease in the gain is that a new unwanted radiation source appears as a result of mounting AMC elements.
FIG. 8 shows, as an example of a result by electromagnetic simulation, frequency characteristics of a directional gain of a patch antenna in which AMC elements are arranged in the area surrounding the patch antenna, in comparison with a patch antenna of the related art in which AMC elements are not arranged in the area surrounding the patch antenna. In the simulation mentioned above, the impedance matching frequency of the patch antenna is generally set to 8 GHz and therefore, the main operating band thereof is also in the vicinity of 8 GHz. It can be seen from FIG. 8 that, although the gain has been improved over that of the patch antenna configuration of the related art at certain frequencies, the gain is lower than that of the patch antenna configuration of the related art in the vicinity of 8 GHz, which is the original operating band.
FIG. 9 shows a simulation result of a radiation pattern at 7.9 GHz in comparison with that of a patch antenna of the related art in which AMC elements are not arranged in the area surrounding the patch antenna. It can be seen from the figure that, in the case of a patch antenna having resonance characteristics, in which AMC elements are mounted in the area surrounding the patch antenna, the gain towards the front of the patch antenna is suppressed on, in particular, an H-plane (φ=90 degrees plane).